Demand for secondary batteries as an energy source has been significantly increased as technology development and demand with respect to mobile devices have increased. Among these secondary batteries, lithium secondary batteries having high energy density, high voltage, long cycle life, and low self-discharging rate have been commercialized and widely used.
However, there is a limitation of rapid reduction of life of the lithium secondary batteries as charge and discharge go on. The limitation is more serious for a battery with long life and a high voltage. The reason is that an electrolyte is decomposed or an active material is deteriorated due to humidity in a battery or other influence, and an increasing phenomenon of internal resistance of a battery is generated. Particularly, in the case of a positive electrode material, if the degeneration of the positive electrode material itself is worsen, the elution of constituent elements of the positive electrode active material increases, thereby rapidly degenerating battery life or disabling the use of a battery at a high voltage.
To solve the above-described limitations, methods for forming a surface treatment layer on the surface of a positive electrode active material have been suggested. Among them, in the case of an aluminum-based surface treatment layer of which stability at a high voltage and in an electrolyte is recognized, coating is applied on the surface of particles in a crystalline state, and uniform coating on the entire active material is difficult. In addition, due to the crystallinity of an aluminum-based compound itself, defects of increasing resistance occur. In addition, in the case of boron (B)-based coating material, uniform coating in an amorphous state is attained, and the transfer of lithium ions from a positive electrode material to an electrolyte is not inhibited. However, since borate-based glass reacts with humidity, a coating layer may not perform its role with the increase of reaction time with an electrolyte.
Accordingly, the development of a positive electrode active material which is capable of solving the above-described limitations and improving the performance of a lithium secondary battery is eagerly required.